That's a pretty big question, which might explain why you haven't gotten much in the way of replies yet! From a high level, there's a few things you'll need to consider.
1. The DL320s appears to be an older machine, from HP's fifth-generation Proliant line. That said, it won't have much in the way of processing capability, will probably be pretty loud, and won't be nearly as power efficient as a machine of even a slightly newer vintage. But it should work to hold data! Assuming the server itself is complete and in working condition (it has a processor, RAM, power supplies, and can actually boot up) then you've got a start. You need to think about where you're going to keep it - obviously it will need to be connected to your network and have power, but you probably don't want it in a living space because of the noise.
2. You'll need hard drives for it if you don't already have them, preferably two for running whatever OS and applications you want on the server and two or more for storage. I think your machine takes 3.5" drives, and you'll need the drive trays/caddies to mount them properly. If it uses the P400 raid controller (which was standard for HP G5 servers) then you can use both SAS and SATA drives, but I think you can only use drives that are 2TB or less. It is a hardware RAID card, so you can build redundancy into the storage by using RAID 1, 10, or 5, but remember that RAID itself is not a backup solution. I would use RAID 1 for the "system" drives and RAID 5 for any media drives. You could use RAID 10 for the media, and it does offer better redundancy and better performance, but you lose more capacity. Wikipedia has a good article on the various RAID levels and their pros/cons. Keep in mind that the drives in an array should be identical, and that if you're not using "enterprise" type drives than RAID can be finicky.
3. How you plan to access your media sets the software requirements. The simplest thing for a beginner is probably to just install some version of Windows on the server and share files/folders from there. iTunes can access shared files over the SMB protocol (which Windows shares use) but you'd have to check for your media player. If it can open network shares directly, it should work. You could always look into something more specific to media sharing/file storage like Plex or file shares with FreeNAS, but those are a little more to set up.
If you really want my honest opinion, while the server might be fun to play around with it might not be the best option for your goals. It is a pretty old piece of equipment, about 10 years now, and will be pretty loud and consume a lot of power while not delivering a lot of performance. For simple media sharing you might be better served by a basic NAS unit; Netgear makes decent ones that can hold a couple of drives for home use. Not to discourage you from playing around with a server, but just make sure that you have realistic expectations! Since you already have the DL320s and it can hold a lot of drives it might be worth it to stay with it, if only for the experience.
Hope that helped and good luck!